1. Field of the Invention
The present invention relates to a semiconductor laser device, and in particular, relates to a semiconductor laser device suitable used for a laser range finder which constitutes robot eyes or laser radar system or the like.
2. Related Arts
Recently, various systems are considered in which a semiconductor laser device is used as systems to measure a distance between two cars going in the same direction to observe a proper distance between such two cars, or to generates an alarm when too close to the car in front, or to brake the car. In these systems, an object in distance 100 m must be detected, this requires the semiconductor laser device an optical output having several tens watts (W) by pulse driving. Such large output semiconductor laser device has a structure as shown in FIG. 12, where an n-GaAs substrate 21 is laminated thereon with an n-GaAs layer 22 and an n-AlGaAs clad layer 23, and further thereon in manner of mesa type with an active layer 24, a p-AlGaAs clad layer 25, and a p-GaAs layer 26. The active layer means a layer which confines an injected carrier and recombines it to convert into a light. In addition, an insulating film 27 and a p-type electrode 28 are formed. The n-GaAs substrate 21 is provided on its backside with an n-type electrode 29 and an Au/Sn layer 30.
In general, several tens watts (W) of large output pulse semiconductor laser device is required to have at least a 100 .mu.m or more in stripe width because of its large output as shown in FIG. 12. On the other hand, it is required that a carrier is confined in a small region and a thickness of the active layer 24 is made an extent of 0.1 .mu.m in order to minimize a threshold current. Thus, in comparison with a wavelength, the large output pulse semiconductor laser device has a larger stripe width, and in addition, the active layer has a decreased thickness. Therefore, a diffraction of the light in the active layer expanding direction (horizontal direction) does not arise, but a diffraction in the active layer thickness direction (vertical direction) arises in a thickness direction thereof. This provides a beam with an ellipse shape expanding in the active layer thickness direction, where is provided an ellipticity (a ratio H/W of a major axis H and a minor axis W of a beam section in FIG. 13) of an extent of approximately 2.5 to 3.2.
An optical lens is used for converging the beam of laser light into a range having a desirous expansion angle, and a laser beam shape emitted may preferably approximate a circular shape as much as possible in view point of easier designing of the lens and miniaturization of the system, an ellipticity less than 2.5 being required.
A general means for obtaining a laser beam having a beam shape approximating a circular form, is disclosed in Japanese Patent Application Laid Open Sho-55-143092 in 1980, where a smaller output semiconductor laser device is used to form a lens portion with a high refractive index on the semiconductor. However, in forming the lens portion within the semiconductor, an additional process is required and this comes to a problem. To solve this, Japanese Patent Application Laid Open Hei-4-151887 teaches the laser beam approximating a circular shape without the lens portion, in which, similarly in the small output semiconductor laser device, optical guide layers (confining layers) having a low refractive index than that of the active layer are formed on upper and lower of the active layer further with the active layer width equal to or more than 0.5 .mu.m and equal to or less than 1.5 .mu.m. The term "optical guide layer" means a layer which acts to confine the light generated in the active layer and to guide the wave.
In a case where the semiconductor laser device disclosed in Japanese Patent Application Laid Open Hei-4-151887 in 1992 is used for a small output (several tens milliwatts (mW)) level of semiconductor laser device (a stripe width: several .mu.m to several tens .mu.m), the laser beam having a beam shape approximating a circular shape is obtained. However, in a case where the laser device is used for larger output pulse semiconductor laser device, the stripe width of which is 100 .mu.m or more, it is found that the laser beam approximating the circular shape can not be obtained only when limiting a width of the active layer and using a structure of putting the active layer between the optical guide layers.
The foregoing designates that generally in the smaller output semiconductor laser device (a stripe width is less than 100 .mu.m), because of the problem in varying of a light emission starting current (a threshold current), a particularly larger thickness is not available for the optical guide layer. It is therefore considered that, change of the beam shape is achieved only by varying a stripe width, or by changing a width of the active layer, or by adding a carrier barrier layer on both sides of the active layer (see Japanese Patent Application Laid Open Sho-61-79288 in 1986). Moreover, since a variation of the light emission starting current is considered to be arisen even in the semiconductor laser device having a stripe width equal to or more than 100 .mu.m, only an ellipticity of an extent of 2.5 to 3.2 as described can be obtained in the large output semiconductor laser device.